1. Field of the Invention
The present invention relates to an optical information reproducing apparatus for reproducing recorded information on an information recording medium such as an optical disk and, more particularly, to an optical information reproducing apparatus optimal for a DWDD (Domain Wall Displacement Detection) scheme.
2. Related Background Art
Recently, with the widespread use of networks, digital cameras, digital video device, and the like, the occasion to handle image data has greatly increased in number, and hence strong demands have arisen for media and apparatuses suited for recording/reproducing moving image data, in particular. Research and development on such media and apparatuses have been vigorously pursed. Of these media and apparatuses, in the field of optical disks, an optical disk and optical disk apparatus have been enthusiastically studied and developed, which are suited for handling digital moving image data. Such a large-capacity optical disk and optical disk apparatus, for example, those based on the domain wall displacement detection scheme disclosed in Japanese Patent Application Laid-open No. 6-290496 are available.
As disclosed in this reference, the domain wall displacement detection scheme is a scheme of irradiating a medium with a beam spot to form a temperature gradient on the medium so as to move a domain wall forming a minute mark, which cannot be resolved with the resolution of an optical head, thereby enlarging the mark in the direction of track moving and reproducing information. By using this domain wall displacement detection scheme, an unprecedented large-capacity system can be built. Reproducing schemes using the magnetic super resolution technique, as well as the domain wall displacement detection scheme, use a temperature distribution on a medium, and hence their characteristics depend on reproducing power, in particular.
While, Japanese Patent Application Laid-open No. 8-63817 discloses a method for a super resolution magneto-optical disk (magnetic super resolution optical disk) apparatus for reproducing a minute mark by using a detection window formed by a temperature distribution on a recording medium. In this method, long and short marks are reproduced in advance, and the amplitude ratio between a long mark reproduction signal level and a short mark reproduction signal level at which jitter is minimized is obtained as a reference value in advance. A reproducing power is then set in accordance with this reference value. According to this reference, a plurality of reproducing power setting regions on which predetermined control data (long and short marks for amplitude ratio calculation) are recorded are formed on a disk, and a reproducing power is set by reading control data from the reproducing power setting regions near reproduction regions.
However, in the above conventional domain wall displacement detection scheme, the magnitude of a reproduction signal is determined by almost instantaneous displacement of the domain wall, and hence the mark length dependence of a signal level is very low. In the domain wall displacement detection scheme, therefore, a large change in amplitude ratio cannot be obtained. For this reason, if the method of setting a reproducing power with reference to the amplitude ratio between a long mark reproduction signal level and a short mark reproduction signal level, disclosed in Japanese Patent Application Laid-open No. 8-63817, is used for the domain wall displacement detection scheme, a reproducing power cannot be accurately set. In addition, setting, on a disk, a plurality of reproducing power regions on which control data is recorded in advance will lead to a capacity loss. Assume that reproducing power setting regions are used for reproducing power control. In the case of a magnetic super resolution optical disk, since a minute mark is detected by using a temperature distribution on the medium, this technique is very sensitive to a reproducing power. Therefore, frequent access to the reproducing power setting regions occurs, resulting in a time loss.
The present invention has been made in consideration of the above problems in the prior art, and has as its object to provide an optical information reproducing apparatus which can accurately set a reproducing power even if an information recording medium using a magnetic super resolution technique that is very sensitive to a reproducing power is used, and is free from any capacity loss and time loss.
In order to achieve the above object, there is provided an optical information reproducing apparatus for reproducing information by irradiating a recording medium with a reproducing light beam, comprising:
a light source for generating the reproducing light beam;
a reproducing circuit for reproducing a reproduction signal by using the light beam; and
an adjusting circuit for adjusting the light beam power,
wherein the adjusting circuit adjusts the beam power to a value larger than the light beam power at which a value obtained by normalizing the reproduction signal with the beam power is maximized.